The invention relates to a process for the parallel preparation of hydrogen, carbon monoxide and a carbon-comprising product, wherein one or more hydrocarbons are thermally decomposed and at least part of the hydrogen-comprising gas mixture formed is taken off from the reaction zone of the decomposition reactor at a temperature of from 800 to 1400° C. and reacted with carbon dioxide to form a gas mixture comprising carbon monoxide and hydrogen (synthesis gas).
CO2 emissions in Germany were about 960 million metric tons of CO2 equivalent in 2010, to which the chemical industry contributes about 5%. From ecological and economic points of view, there are big incentives in the chemical sector to reduce CO2 emissions by alteration of the raw materials basis, low-CO2 production technologies, optimization of energy consumption and utilization of process-related CO2 to produce large-volume basic chemicals. Suitable basic chemicals are, for example, hydrogen and synthesis gas. The latter forms the ideal interface to existing petrochemical processes for the production of, for example, methanol, dimethyl ether or Fischer-Tropsch products. The worldwide demands for hydrogen and synthesis gas are at present 50 million metric t/a and 220 million metric t/a, respectively.
Steam reforming is at present the most widespread process for producing synthesis gas having a hydrogen to carbon monoxide ratio of from 2.5 to 3.0 from light hydrocarbons. The proportion of hydrogen can be increased still further at the expense of the carbon monoxide content by means of the additional complication of carrying out a shift stage. The feedstocks natural gas, liquefied gas or naphtha are endothermically reacted with steam in catalytic tube reactors to give synthesis gas having a high hydrogen content. Process heat and flue gas heat are utilized for steam generation.
The preparation of synthesis gas having a hydrogen to carbon monoxide ratio of from 1.0 to 2.0 by steam reforming is problematical. This ratio of H2/CO is of great importance for downstream utilization in the field of fuels and chemicals and is a particular challenge for steam reforming since this requires catalysts which have particular stability against carbonization. Alternative processes for preparing synthesis gas having an H2/CO ratio of from 1.0 to 2.0 are autothermal reforming of natural gas and partial oxidation. Both processes are employed industrially, but require the use of pure oxygen which is obtained by fractionation of air. For example, the partial oxidation of natural gas in the feed requires a theoretical molar O2:CH4 ratio=1:2. The highly energy-consuming cryogenic fractionation of air is therefore a considerable cost driver for the latter two processes.
Important consumers of synthesis gas are, inter alia, the synthesis of methanol (hydrogen to carbon monoxide ratio of 2:1), the oxo process for preparing alcohols and aldehydes from olefins (hydrogen to carbon monoxide ratio of 1:1) and the Fischer-Tropsch synthesis to produce alkanes (hydrogen to carbon monoxide ratio of 2:1) or the Fischer-Tropsch synthesis to produce olefins (hydrogen to carbon monoxide ratio of 1:1).
In addition, synthesis gas serves as hydrogen source for the synthesis of ammonia. Here, a hydrogen-rich synthesis gas from steam reforming (hydrogen to carbon monoxide ratio of 3:1) is enriched further in hydrogen in a water gas shift reaction stage. Here, CO is converted into CO2 which is subsequently separated off. Pure hydrogen is obtained in this way.
A further disadvantage of catalytic steam reforming is the susceptibility to catalyst poisons such as sulfur. To protect the catalyst from these, the feed gas has to be purified in a complicated manner in preceding process stages. Organic sulfur compounds, for example mercaptans or thiophenes are hydrogenated beforehand over Co—Mo or Ni—Mo catalysts to form hydrogen sulfide. Hydrogen sulfide is, for example, reacted over ZnO which is converted into ZnS as a result and has to be replaced regularly. Furthermore, heat integration of steam reforming is incomplete, and only about 50% of the heat produced is utilized effectively for the reaction. Furthermore, the synthesis gas produced in this way has a quite high CO2 footprint of about 40 kgCO2/100 kgsynthesis gas.
The reforming of methane and carbon dioxide, an alternative way of preparing synthesis gas, is of great economic interest since this process offers the opportunity of preparing synthesis gas as important basic chemical utilizing carbon dioxide as starting material. Accordingly, carbon dioxide which is obtained as waste product in numerous processes could be bound by means of a chemical route. This offers the possibility of reducing carbon dioxide emissions into the atmosphere. Here too, catalyst development is a particular challenge since catalysts which have particular stability against carbonization are likewise required here.
Such processes are known, for example, from the patent applications US 2009/203519 and US 2011/089378. Both patent applications describe processes in which methane and carbon dioxide are passed over a catalyst and reacted by dry reforming. Owing to the Boudouard equilibrium and the thermal decomposition of methane, carbon is formed and deposits, inter alia, on the catalyst and poisons the latter, so that the catalyst has to be replaced or regenerated at regular intervals.
A further alternative way of preparing synthesis gas using carbon dioxide as starting material is the reverse water gas shift (RWGS) reaction. Activation of CO2 by means of hydrogen via the RWGS reaction leads to carbon monoxide and water and is endothermic with an enthalpy of reaction of 41 kJ/mol under standard conditions. According to the thermodynamic equilibrium, temperatures of greater than 500° C. are necessary for substantial CO formation since methanation otherwise occurs preferentially at temperatures below 500° C. Only laboratory studies are known for the reaction (Luhui, W.; Shaoxing, Z.; Yuan, L.: Reverse water gas shift reaction over co-precipitated Ni—CeO2 catalysts. Journal of Rare Earths 2008, 26, 66-70; Yablonsky, G. S.; Pilasombat, R.; Breen, J. P.; Bruch, R.; Hengrasmee, S.: Cycles Across an Equilibrium: A Kinetic Investigation of the Reverse and Forward WGS Reaction over a 2% Pt/CeO2 Catalyst (Experimental Data and Qualitative Interpretation). Chem. Eng. Sci. 2010, 65, 2325-2332; Jess, A.; Kaiser, P.; Kern, C.; Unde, R. B.; Olshausen, C.: Considerations concerning the Energy Demand and Energy Mix for Global Welfare and Stable Ecosystems. Chemie Ingenieur Technik 2011, 83, 1777-1791).
Industrially, the RWGS reaction has hitherto not been carried out since no inexpensive hydrogen source which has a small or acceptable CO2 footprint and could allow the RWGS reaction to be carried out economically at high temperatures has been available up to the present day.
Competitive processes for preparing hydrogen are still steam reforming processes. Although these processes have an inherent price advantage which is reflected in the hydrogen price, hydrogen production is coupled with a high emission of carbon dioxide. For this reason, it is not feasible for technical and economic reasons to use the hydrogen produced in steam reforming for the hydrogenation of carbon dioxide in a further endothermic process step. Alternative hydrogen sources based on renewable raw materials can overcome the coupling of production to carbon dioxide from hydrogen production, but it has to be taken into account here that the high temperature level of the RWGS reaction required for synthesis gas production still requires a high energy input.
Although high-temperature RWGS reactions would be effective for achieving a high conversion of carbon dioxide and advantageous for suppressing methanation and carbon formation as undesirable secondary reactions, this has not been studied to any great extent in the past. Carrying out the reaction at this high temperature level requires a very high engineering outlay for heat input or energetically favorable coupling with a high-temperature source. According to the present-day prior art, only furnaces comparable to steam reformers come into question for this high-temperature energy input. However, in these processes only about 50% of the quantity of heat generated can be taken up by the endothermic reaction. The excess heat thus has to be removed in a complicated network of heat exchangers and recirculated in the process, for example for preheating the feed streams.
There are indications in the literature, e.g. Kreysa, CIT 80 (2008), 901-908, that hydrogen can be produced with a small CO2 footprint by means of thermal decomposition (pyrolysis) of hydrocarbons in a fluidized bed composed of carbon-comprising granules. The countercurrent transport of the gaseous reaction mixture and the carbon-comprising granules gives a process having integrated heat recirculation. However, an efficiency of the heat integration, defined as ratio of the heat consumed by the endothermic reaction to the heat introduced of at best 83% can be achieved by means of this process. The reason for this is the ratio of the heat capacities of the starting materials and of the products of pyrolysis: for example, the average specific heat capacity of the methane used in the range from 100° C. to 1200° C. is 4.15 J/g/K. The mixture of H2 and C produced therefrom has an average specific heat capacity in the range from 100° C. to 1200° C. of 5.02 J/g/K. This ratio of the heat capacities fixes the maximum achievable efficiency of heat integration.
It was therefore an object of the present invention to provide a route for the parallel preparation of hydrogen, carbon monoxide and/or a solid carbon-comprising product with a small CO2 footprint under economically attractive boundary conditions for the chemical industry.
A further object was to discover suitable process conditions which allow the reverse water gas shift reaction to be carried out economically and in a technically attractive manner at high temperatures.
A further object was to provide a process for synthesis gas production which can not only produce a fixed hydrogen to carbon monoxide ratio but also enables the hydrogen to carbon monoxide ratio to be set as required.
A further object was to provide a process for the thermal decomposition of hydrocarbons into carbon and hydrogen and conversion of the hydrogen into synthesis gas by means of carbon dioxide, which process has virtually complete heat integration, i.e. a higher efficiency than the individual reaction of the thermal decomposition of hydrocarbons to carbon and hydrogen.
The coupling of the RWGS reaction with a high-temperature process whose excess heat is utilized as driving force for the RWGS reaction appears to be advantageous. The desired heat integration makes it possible to create a scenario in which the heat flow from the high-temperature process is utilized to operate the RWGS reaction at a high temperature level.